WLAN access scheduling control

ABSTRACT

The present invention provides a system for scheduling access times for transmitting data traffic in a WLAN ( 10 ). The data traffic includes both isochronous streams and asynchronous bursts characterized by widely varying parameter values. A succession of service periods is generated, each having adjustable length and including two or other number of primary access intervals. The service periods may also be adapted to include one or more asynchronous burst periods, each immediately following a primary access interval. A hybrid coordinator ( 30 ) located at the AP ( 12 ) of the WLAN operates to schedule access times for each ischronous stream only during a primary access interval, or an immediately following extension period. Asynchronous bursts are scheduled for access times only during asynchronous burst periods. Thus, an access time scheduling is made available that can readily adapt WLAN access to continually changing data traffic conditions, and thereby provide QoS by scheduling access times accordingly.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention generally pertains to schedulingtransmission times for data traffic in a wireless local area network(WLAN) and, more particularly, to scheduling access times for traffic ofdifferent types, including isochronous streams and asynchronous bursts.

BACKGROUND OF THE INVENTION

[0002] WLAN provides an Ethernet-like channel that uses wireless media(e.g., radios) instead of wires or cables to enable communicationbetween or amongst computers and other types of electronic devices.While providing mobility and portability, WLANs avoid the effort andcosts involved in running and maintaining cables, and thus are becomingincreasingly popular as a communication medium. A WLAN can have multiplestations and access points (APs), where an AP serves to attach orconnect respective stations to an external network and to one another.Generally, a WLAN has several protocol layers—among which are a physical(PHY) layer and a medium access control (MAC) sub-layer. A MAC,comprising hardware and software, is located at each station and AP, andcontrols access to the medium. A transmission from an AP to a station isreferred to as a downlink, a transmission from a station to the AP isreferred to as an uplink, and a transmission between stations isreferred to as a sidelink.

[0003] Because all transmissions within a WLAN must share a singlechannel or communication medium, conflicts or collisions are likely tooccur whenever different traffic streams and bursts simultaneouslyarrive at one or more transmission points (i.e., stations and the APcollectively) and need to access the shared medium for transport toother transmission points. Accordingly, several approaches have beendeveloped to mediate access between competing traffic streams andbursts, and to provide access scheduling that avoids collisions over themedium.

[0004] In an approach commonly known as Distributed Coordinator Function(DCF), the station must sense the medium before a data frame is sentfrom a station. If the medium is found to be idle for at least a DCFInter Frame Space (DIFS) period of time, the frame is transmitted.Otherwise, a back-off time B (measured in time slots) is chosen randomlyin the interval [0-CW], where CW is the so-called contention window.After the medium has been detected as being idle for at least a DIFS,the back-off timer is decremented for each time slot the medium remainsidle. When the back-off timer reaches 0, the frame is transmitted. Upondetection of a collision, a new back-off time is chosen using a CW thatis double the previous one, and the back-off procedure starts over.

[0005] An enhancement of DCF, commonly known as EDCF, has been developedto provide service differentiation. With EDCF, once a station has gainedaccess to the medium, it can be allowed to send more than one framewithout contending for the medium again. More particularly, afterobtaining access to the medium, a station is allowed to send as manyframes as it wishes, as long as the total access time does not exceed aspecified limit.

[0006] Furthermore, a WLAN environment may be fairly hostile to radiosignals, due to noise and interference as well as a number of otherdisruptive factors. Accordingly, it is common practice to have arecipient acknowledge a transmission, and for a sender to resend atransmission upon failure to receive an acknowledgement signal after aperiod of time.

[0007] Moreover, WLAN transmissions may be of different traffic typesincluding both isochronous streams and asynchronous bursts. Isochronousstreams have quasi-periodic data arrivals and include voice and videostreams. Asynchronous bursts are essentially traffic arrivals in burstwith low duty cycles and include file transfers and interactive data.The streams and bursts can be of Continuous Bit Rate (CBR) or VariableBit Rate (VBR), can have respectively different data rates, and can bedifferent from one another in regard to data rates, delay and jitterrequirements, acknowledgement policies and other parameters. Also, whilethere may be distinct advantages in using long data frames intransmissions, there may be other advantages in using shorter frames.

[0008] Because of the widely varying characteristics of WLAN traffic,conventional techniques for scheduling access to a WLAN, such as EDCF,have certain significant limitations. For example, if a station having anumber of long data frames to send gained access to the medium in anEDCF arrangement, it could transmit its long frames until it wasfinished, while a station with a shorter frame was forced to wait. Thiswould occur despite the desirability of interposing longer and shortertime frames during successive time periods, such as to reduce jitterbetween the adjacent longer frames. It might also be desirable to givepriority to certain signals in scheduling access (e.g., to enablereal-time critical data frames to be sent before non real-time criticaldata frames). Furthermore, EDCF is a contention based access method,resulting in heavy collisions due to simultaneous transmissions fromstations as the number of contending stations increases. Althoughscheduling methods for contention-free transmissions exist, none of themappear to addresses shared medium access by downlink, uplink, andsidelink, as is the case in a WLAN.

[0009] As a result, there is a need for a system for schedulingcontention-free access times for data transmissions in a WLAN accordingto selected Quality of Service (QoS) requirements, such as data ratesand delay bounds.

SUMMARY OF THE INVENTION

[0010] The present invention provides a versatile system for schedulingaccess times in a WLAN, wherein the availability of access can beadapted to meet dynamic needs and priorities of user traffic. Thepresent invention thus provides selected QoS levels in scheduling accessto the wireless medium for respective WLAN traffic data. As usedhereinafter, “scheduling access” and “scheduling access times” meanallocating specific times to stations and APs of a WLAN for respectivedata transmissions.

[0011] Embodiments of this invention can be used with data transmissionsof different traffic types, including isochronous streams andasynchronous bursts, and with streams and bursts having data frames ofwidely varying lengths, data rates and other parameter values. Moreover,embodiments of this invention provide criteria for resolving accessconflicts between streams and bursts and between different streams.Embodiments of the present invention provide determination of accesstimes for uplink and sidelink transfers, as well as downlink transfers.Embodiments of this invention also reclaim scheduled access times thatcannot be used as originally intended.

[0012] One embodiment of the present invention provides a method forscheduling access for transmission of data frames from a firsttransmission point in a WLAN to a second transmission point therein,wherein data frame traffic includes both isochronous streams andasynchronous bursts, and is characterized by variations in specifiedparameter values. The method includes defining a succession of serviceperiods, each service period including a specified number of isochronousepochs, and adapting at least one of the service periods to include atleast one asynchronous epoch, each asynchronous epoch immediatelyfollowing an isochronous epoch of the same service period. The methodfurther includes scheduling access for the isochronous streams duringthe asynchronous epochs, and scheduling access for the asynchronousbursts only during the asynchronous epochs.

[0013] In another embodiment, each of the service periods includes twoof the isochronous epochs, and further includes one, two or none of theasynchronous epochs, selectively. Each of the isochronous epochs mayinclude a primary access interval, and may be readily adapted to includean extension period. One or more of the isochronous streams arescheduled for access during each of the primary access intervals, andone or more of the scheduled isochronous streams is provided withadditional access time during each of the extension periods.

[0014] Another embodiment of the present invention provides apparatusfor transmitting data frames from a first transmission point in a WLANto a second transmission point therein, wherein data frame trafficincludes both isochronous streams and asynchronous bursts. The apparatuscomprises a first component located at one of the transmission pointsfor scheduling access times for data transmission during a succession ofservice periods. Each service period includes a specified number ofisochronous epochs, and at least some of the service periods include oneor more asynchronous epochs. The first component may comprise, forexample, a hybrid coordinator for the MAC located at an AP of the WLAN.The apparatus may further include a second component—a transceiverlocated at each station and AP, acting in cooperation with the firstcomponent to successively transmit and receive the isochronous streamsand asynchronous bursts.

[0015] Other features and advantages of the present invention will beapparent to those of ordinary skill in the art upon reference to thefollowing detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a better understanding of the invention, and to show by wayof example how the same may be carried into effect, reference is nowmade to the detailed description of the invention along with theaccompanying figures in which corresponding numerals in the differentfigures refer to corresponding parts and in which:

[0017]FIG. 1 is a schematic diagram showing basic components of a WLANdisposed for operation in accordance with one embodiment of the presentinvention;

[0018]FIG. 2 is a block diagram showing the access point (AP) of FIG. 1in further detail;

[0019]FIGS. 3-4 are timing diagrams generally illustrating access timescheduling in an embodiment of the present invention;

[0020]FIGS. 5-6 are timing diagrams illustrating an embodiment of thepresent invention wherein extended access time is provided forisochronous streams; and

[0021]FIGS. 7-8 are timing diagrams illustrating one embodiment of thepresent invention wherein access time is provided for asynchronousbursts.

DETAILED DESCRIPTION OF THE INVENTION

[0022] While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts, whichcan be embodied in a wide variety of specific contexts. The inventionwill now be described in conjunction with any memory. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not limit the scope of the invention.

[0023] The present invention provides a versatile system for schedulingaccess times in a WLAN. According to the present invention, availabilityof access can be adapted to meet dynamic needs and priorities of usertraffic. The present invention thus provides selected QoS levels inscheduling access to the wireless medium for WLAN traffic data.

[0024] Referring to now FIG. 1, a WLAN 10 adapted to operate inaccordance with one embodiment of the present invention is depicted.WLAN 10 includes an AP 12 and stations 14 and 16, by way ofillustration. Operational WLANs likely would include many more stations.FIG. 1 also shows AP 12 connected to a distributed system 18.Accordingly, a communication path can be established from AP 12 toanother WLAN 20 through an AP 22, which is also connected to distributedsystem 18. Respective stations of WLAN 10 are thereby able tocommunicate with the station 24 of WLAN 20.

[0025] A downlink transmission in WLAN 10 is directed from the AP 12 toa station, an uplink transmission is directed from a station to the AP12, and a sidelink transmission is directed from one station to another.WLAN 10 is provided with a MAC sub-layer. Access for data transmissionsin WLAN 10 is scheduled in accordance with one embodiment of theinvention, as described hereinafter. Such scheduling accommodates datatraffic in the form of both isochronous streams and asynchronous bursts,and readily adapts to continual variations in data amount and otherparameters. Thus, selective QoS is provided by scheduling access fordata transmission in accordance with embodiments of the invention.

[0026] For a downlink transfer, AP 12 transmits a data frame that may beQoS Data, or a data frame combined with a poll frame, such as QoSData+CF-Poll or QoS Data+CF-Ack+CF-Poll. For an uplink or sidelinktransfer, AP 12 transmits a poll frame, such as a QoS CF-Poll, a QoSCF-Ack+CF-Poll, or a QoS Data+CF-Ack+CF-Poll frame. A poll frame grantsa specific access time to a polled station for the transmission of datafrom this station into the wireless medium within that access time. Whenappropriate, an acknowledgement frame, such as a QoS CF-Ack, is sentback to the sender of the data frame by the addressed recipient of thatdata frame, within a Short Inter-Frame Space (SIFS) of the end of thedata frame to acknowledge receipt of the data frame.

[0027] Referring to FIG. 2, there is shown AP 12 provided with a hybrid(or access) coordinator 30. Hybrid coordinator 30 is the component ofthe MAC for WLAN 10 that is used to schedule access for datatransmissions within WLAN 10. Hybrid coordinator 30 has an eCPU 32, andschedules access in accordance with one embodiment of the presentinvention, as described hereinafter.

[0028]FIG. 2 further shows AP 12 provided with MAC hardware memory 34and a host memory 36. For a downlink transfer, eCPU 32 transfers a dataframe of the next isochronous stream or asynchronous burst scheduled foraccess from host memory 36 to the MAC hardware memory 34, at a timeT_(D) before the expected starting time of that data frame. For uplinkand sidelink transfers in WLAN 10, eCPU 32 at the hybrid coordinator 30prepares a poll frame in the MAC hardware memory 34 at a time T_(P)before the expected starting time of such poll frame. If there is datato be sent along with the poll frame, the transfer procedure describedabove in connection with downlink transfer is followed. Frames aretransmitted from memory 34 by transceiver 38 into a wireless medium 39.

[0029] Referring to FIG. 3, there is shown the time for schedulingaccess within WLAN 10 divided into a succession of service periodsT_(SP,n), T_(SP, n+1). There is further shown a set of time hierarchiesin each service period, including two isochronous epochs T_(IE1) and_(IE2), which are time intervals when isochronous streams may bescheduled for access. Each of the isochronous epochs is further divided,T_(IE1) into T_(PP1) and T_(PE1), and T_(IE2) into T_(PP2) and T_(PE2).T_(PP1) and T_(PP2) are the primary access intervals for isochronousstreams, and T_(PE1) and T_(PE2) are the secondary or extension accessintervals therefor, as described hereinafter.

[0030] Referring further to FIG. 3, each service period is also shown toinclude asynchronous epochs T_(AE1) and T_(AE2), the access intervalsfor asynchronous bursts. Time may also be made available during theseepochs, if desired, for implementing an (E)DCF-like contention mediationscheme to send short data frames and/or reserve access times by stationshaving new asynchronous bursts to send. Accordingly, asynchronous epochsT_(AE1) and T_(AE2) are further divided, into T_(AP1) and T_(C1), andT_(AP2) and T_(C2), respectively. T_(AP1) and T_(AP2) are the intervalsallocated to provide contention-free access for asynchronous bursts,whereas T_(C1) and T_(C2) are the access intervals for contentionmediation.

[0031] In one embodiment of the present invention, active isochronousstreams of data (i.e., streams waiting for access times for transmissionin the WLAN) are respectively organized into two Stream Lists L_(S1) andL_(S2). The streams in L_(S1) and L_(S2) have access times scheduledduring intervals T_(PP1) and T_(PP2), respectively, for each serviceperiod, and the streams are placed on the respective lists so thatT_(PP1) is approximately equal to T_(PP2). Streams are ordered in eachlist in ascending delay bounds, so that the stream with the smallestdelay bound (T_(DB)) is first on the list. Moreover, the streams in eachlist are updated upon change of an isochronous stream, have thescheduled access times repeated in each service period, and may haveextended access times during extension intervals T_(PE1) and T_(PE2).

[0032] In one algorithm for constructing stream lists L_(S1) and L_(S2),the indexes i of active streams are ordered in ascending value ofT_(DB), to provide i1, i2, . . . ip. Access times T_(i1), T_(i2) . . .T_(ip) are then determined for respective active streams, for eachsuccessive service period T_(SP). L_(S1) is set to i1, L_(S2) is set toi2, and T_(PP1) and T_(PP2) are made equal to T_(i1) and T_(i2),respectively. Then, the following procedure is continually looped:

[0033] For ij=i3 to ip:

[0034] if T_(PP1) is less then T_(PP2), then index ij is appended toL_(S1) and T_(PP1) is set to T_(PP1)+T_(ij);

[0035] else, index ij is appended to L_(S2) and T_(PP2) is set toT_(PP2)+T_(ij).

[0036] Hereinafter, as shown in FIG. 3, contents of Stream Lists L_(S1)and L_(S2) are represented as (i1, i2, i3) and (j1, j2, j3),respectively.

[0037] Referring further to FIG. 3, Burst List L_(B) is depicted,containing all active asynchronous bursts. The bursts in L_(B) areordered in ascending arrival times of the bursts and have userpriorities A-D. Access times in the next following asynchronous intervalT_(AP1) or T_(AP2) are allocated to the bursts in List L_(B), in a firstcome first serve (FCFS) order, and the List L_(B) is updated uponarrival or removal of each asynchronous burst.

[0038] In one algorithm for constructing Burst List L_(B), the indexesof active bursts are ordered in ascending burst arrival times to providev1, v2 . . ., vw. Burst vj, the burst at the head of List L_(B), isremoved from L_(B) when its allocated access time begins, regardless ofwhether the burst will be completely transferred within such accesstime. Burst vk is appended to L_(B) upon arrival. Arrival of such aburst is indicated by a nonzero Remaining Data Size or by an AdditionalAccess Time of the Transmit (TX) Descriptor for the burst vk. Anexisting burst that was removed from L_(B) when its access time began isappended to L_(B) following that access time, if there is an indicationthat the burst has more buffered data at that time. A new burst isappended to L_(B) when there is found an indication that the new bursthas arrived at a station or the AP for transfer. For uplink/sidelinktransfer the indication comes from a QoS Data or QoS Null frame sent inresponse to a poll in a previous primary access interval or extensionperiod or by means of contention in a previous (E)DCF section.

[0039] In the timing arrangement shown in FIG. 3, the time T_(SP) for aservice period can be determined from the following:

T _(SP)=Max[T _(SP,Min), Min(T _(SP,Max) , T _(Beacon) , T _(1AI,i) , T_(DB,i) , T _(JB,i))],

iεall active isochronous streams  Eqn. (1)

[0040] In the relationship shown in Eqn. (1), T_(SP, Min) andT_(SP, Max) are pre-set lower and upper limits on T_(SP), and areusefully selected to be 12 ms and 20 ms, respectively, but are notlimited thereto. T_(1AI,i) and T_(JB,i) are the inter-arrival intervaland the jitter bound for the ith isochronous stream, respectively.T_(Beacon) is the period of a synchronizing beacon frame transmitted bythe AP of WLAN 10. Thus, in order to determine the value of a serviceperiod T_(SP), the lowest value of a number of parameters withrespective to all the admitted isochronous streams as indicated in Eqn(1) is first determined. T_(SP) is then selected to be either suchlowest value or T_(SP, Min), whichever is greater.

[0041]FIG. 4 depicts streams i1, i2, and i3 scheduled for access timesT_(i1), T_(i2) and T_(i3), respectively, during the primary accessinterval T_(PP1) of each service period. Similarly, streams j1, j2, andj3 are scheduled for access times T_(j1), T_(j2), and T_(j3) during theprimary access interval T_(PP2) of each service period. The target starttime of T_(PP1) coincides with the start of its corresponding serviceperiod T_(SP). The target start time of T_(PP2) is T_(SP-PP2), followingthe commencement of the corresponding service period.

[0042] T_(PP1) and T_(PP2) are more specifically provided by thefollowing relationships:

T _(PP1) =Σ Ti, iεL _(S1)  Eqn. (2a)

T _(PP2) =Σ Tj, jεL _(S2)  Eqn. (2b)

[0043] Referring further to FIG. 4, streams i1, i2, and i3 are depicted,actually using access times T′_(i1), T′_(i2), and T′_(i3), respectively,during intervals T′_(PP1) of the consecutive service periods. T′_(PP1)and T′_(PP2) are more specifically shown as follows, where T′_(PP1) isthe access time that is actually used out of the scheduled accessperiod:

T′ _(PP1) =Σ T′ _(i) , iεL _(S1)  Eqn. (3a)

T′ _(PP2) =Σ T′ _(j) , jεL _(S2)  Eqn. (3b)

[0044] For certain embodiments of the present invention, the followingconventions may be applied:

[0045] (1) T′_(i) may always be less then or equal to T_(i);

[0046] (2) T′_(i) may start earlier then scheduled, and be greater thanT_(i) only if the period of T′_(i) ends no later than the targeted orscheduled end of T_(i).

[0047] The present invention thus provides access intervals T_(PE1) andT_(PE2), which can be used to extend the access time allocated toisochronous streams in a service period T_(SP). FIG. 5 shows extendedinterval T_(PE1), scheduled out of the isochronous epoch T_(IE1) of aservice period, and further shows interval T_(PE2) scheduled thereafter.If T _(i) is the extended access time allocated in interval T_(PE1) orT_(PE2) for stream i in a service period, then:

T _(PE1) =ΣT _(i) , iεL _(S1)  Eqn. (4a)

T _(PE2) =ΣT _(j) , jεL _(S2)  Eqn. (4b)

[0048]T _(i) is the minimum value of either T_(i−), or the quantity(T_(n)+T_(i)−T′_(j)), where T′_(i) is the actual access time used basedon T_(i), T_(i−) is the additional access time needed after T′_(i) totransfer the remaining buffered data of stream i, and T_(n) is theeligible access time not yet used by stream i at the beginning ofT_(SP, n). T_(n) recursively determines T_(n+1), T_(n+1)=T_(n)+T_(i)−T′_(i)−T′_(i), for use in similar calculations for the nextservice period.

[0049] Referring now to FIG. 6, T′_(PE1) and T′_(PE2) are the amounts oftime actually used for extended access times for service periodT_(SP, n). T′_(PE1), shown in timing diagram (A), is the sum of T′_(i1)and T′_(i2), the access time extensions actually used for streams i1 andi2, respectively. T′_(PE2), in timing diagram (B), is the sum ofT′_(i1), T′_(i2), and T′_(i3), the access time extensions used forstreams j1, j2 and j3, respectively. If T′₁ is the extended access timeactually used out of the allocated extended access time T _(i), T′_(PE1)and T′_(PE2) have the following relationships:

T′ _(PE1) =ΣT′ _(i) , iεL′ _(S1)  Eqn. (5a)

T′ _(PE2) =ΣT′ _(j) , jεL′ _(S2)  Eqn. (5b)

[0050] In one embodiment of the invention, the actually used extendedaccess time T′_(i) can be less than or equal to the allocated extendedaccess time T _(i), but can never be greater. Also, the extended accesstime List L′_(S1) for service period T_(SP,n) must be limited to theelements in the front of L_(S1) such that the actual extension intervalT′_(PE1) will end before the targeted or scheduled start of the nextT_(PP1) by |T_(PP2)| |T_(PP2)| is the length of the second scheduledprimary access interval for service period T_(SP,n), and the nextT_(PP1) is the first scheduled primary access interval for serviceperiod T_(SP,n+1).

[0051] In like manner, list L′_(S2) for service period T_(SP,n) must belimited to the elements in the front of L_(S2) so that T′_(PE2) will endbefore the targeted start of T_(PP2) for the next service periodT_(SP,n+1) by |T_(PP1)|. |T_(PP1)| is the length of the first scheduledprimary access interval for service period T_(SP,n+1). It is seen fromtiming diagram (B) of FIG. 6 that access time extensions associated witha service period can be extended into the next following service period,provided the above criteria are met. This feature enhances flexibilityand efficiency in the access time scheduling process.

[0052] Referring now to FIG. 7, each service period comprises a burstaccess interval T_(AP1) lying within an asynchronous epoch T_(AE1) whichfollows the actually used isochronous epoch T′_(IE1), and another burstaccess interval T_(AP2) lying within an asynchronous epoch T_(AE2)occurring after the actually used isochronous epoch T′_(IE2). If T_(v)is the access time allocated in the next interval T_(AP1) or T_(AP2) forthe burst v, then access is given to the bursts in L_(B) sequentiallyand takes place in T_(AP1) or T_(AP2), whichever comes next. Morespecifically,

T _(APK) =ΣT _(v) , vεL _(B)  Eqn. (6)

[0053] T_(APK) must end at a time T_(C-MIN) before the end of the nextscheduled primary access interval, whether T_(PP1) or T_(PP2). T_(C-MIN)is the minimum interval provided for contention access.

[0054] It may be desirable to establish user priority for respectivebursts v. For example, in one arrangement each burst is allocated anaccess time T_(V), where V=A, B, C or D, A<B<C<D. The access times forbursts of respective priorities are then T_(A)=W_(A)×T_(O),T_(B)=W_(B)×T_(O), T_(C)=W_(C)×T_(O), and T_(D)=W_(D)×T_(O). The W_(v)are weighting values, such as W_(A)=1, W_(B)=2, W_(C)=3, and W_(D)=4.T_(O) is a time period that may be derived from the weighting values andfrom the numbers of bursts of respective priorities at the beginning ofeach burst interval T_(AP1) or T_(AP2).

[0055] Referring now to FIG. 8, timing diagram (A) depicts no actuallyused burst interval T′_(AP1) or T′_(AP2) in service period T_(SP, n)which was completely used by isochronous streams. In timing diagram (B),service period T_(SP, n+1) comprises no actually used burst intervalT′_(AP1), but an actually used burst interval T′_(AP2), the amount ofaccess time used for bursts b8 and c5. More generally, if T′_(v) is theaccess time actually used out of allocated time T_(v), then theaggregate of the actual access times for cumulative bursts in a givenburst interval T′_(AP1) or T′_(AP2) is given by:

T′ _(APK) =ΣT′ _(v) , vεL′ _(B)  Eqn (7)

[0056] In one embodiment of the present invention, T′_(APK) can be lessthan or equal to the allocated interval T_(APK), but it can never begreater. The burst list L′_(B) must be limited so thatT′_(APK)+T_(C-MIN) ends before the scheduled start of the next primaryaccess interval T_(PP1) or T_(PP2).

[0057] In implementing the scheduling of access times for streams andbursts, the indexes of the streams in L_(S1) and L_(S2) and/or of thebursts in L_(B) are part of the parameters of the TX Descriptors of thecorresponding streams or bursts in the data structure. There isprearranged certain memory space for storing the TX Descriptors ofactive streams and bursts.

[0058] The TX Descriptor of a stream or burst contains the RemainingData Size or Additional Access Time. For down link transfers, this itemis updated following an arrival of new data to the stream or burst, orfollowing a successful transmission of a frame of the stream or burst.For uplink and sidelink transfers, this item is updated following areception, by the AP, of a frame of the stream or burst, the receivedframe indicating the Remaining Data Size or Additional Access Timethereof. The TX Descriptor further contains scheduling parameters suchas mean data rate and delay bound. A station maintains TX Descriptorsfor its own uplink and sidelink streams and bursts. The AP maintains alist of TX Descriptors for downlink as well as uplink and sidelinkstreams and bursts, the TX Descriptors for uplink and sidelink streamsand bursts being reduced to contain the Remaining Data Size orAdditional Access Time plus a few other parameters. The TX Descriptorsat the AP will also have a parameter indicating whether it is fordownlink or for uplink/sidelink, and a parameter indicating whether itis for a stream or for a burst.

[0059] One embodiment of the present invention comprises a procedure fordetermining whether or not a new stream should be admitted for accesstime scheduling. In such a procedure, the total access time used duringthe isochronous epochs of each of a number of previous service periodsis tracked or monitored, and the access time required by the new streamis calculated. This information is then used to generate a ratio that isused to determine whether a new stream is to be admitted or rejected.

[0060] The embodiments and examples set forth herein are presented tobest explain the present invention and its practical application and tothereby enable those skilled in the art to make and utilize theinvention. However, those skilled in the art will recognize that theforegoing description and examples have been presented for the purposeof illustration and example only. The description as set forth is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching without departing from the spirit and scope of thefollowing claims.

What is claimed is:
 1. A method for scheduling access times fortransmitting data frames from a first transmission point in a WLAN to asecond transmission point therein, wherein data frame traffic includesboth isochronous streams and asynchronous bursts and is characterized byvariations in specified parameter values, the method comprising thesteps of: defining a succession of service periods, each service periodincluding a specified number of isochronous epochs; adapting at leastone of the service periods to include at least one asynchronous epoch,each asynchronous epoch immediately following an isochronous epoch ofthe same service period; and scheduling the access times for theisochronous streams only during the isochronous epochs, and for theasynchronous bursts only during the asynchronous epochs.
 2. The methodof claim 1 wherein each of the service periods includes two of theisochronous epochs, and further selectively includes one, two or none ofthe asynchronous epochs.
 3. The method of claim 2 wherein said methodfurther comprises incorporating each of the isochronous streams into oneof two stream lists, each stream list corresponding to one of theprimary access intervals of each service period.
 4. The method of claim1 wherein each of the isochronous epochs includes a primary accessinterval, and is adaptable to include an extension period, at least oneof the isochronous streams being scheduled for access time during eachof said primary access intervals, and at least one of the scheduledisochronous streams being provided with additional access time duringeach of the extension periods.
 5. The method of claim 1 wherein each ofthe isochronous epochs includes a primary access interval and anextension period, the extension period extendable into the nextfollowing scheduled isochronous epoch to the extent that sufficientaccess time is left to accommodate the next following scheduled primaryaccess interval.
 6. The method of claim 4 wherein the actual access timeduring one of the primary intervals may start earlier, but may not endlater, than the corresponding scheduled access time, and the amount ofaccess time actually used during one of the extension periods may notexceed the corresponding scheduled access time.
 7. The method of claim 1wherein each of the asynchronous epochs includes an asynchronous burstperiod and a contention period, at least one of the asynchronous burstsgiven access time during each of the burst periods.
 8. The method ofclaim 7 wherein each of the asynchronous epochs ends before thescheduled commencement of the next following primary access interval. 9.The method of claim 7 wherein the amount of access time actually usedduring one of the burst periods may not exceed the correspondingscheduled access time.
 10. The method of claim 7 wherein each of theasynchronous bursts is incorporated into a burst list.
 11. The method ofclaim 7 wherein respective asynchronous bursts are allocated accesstimes on the basis of specified weighting values.
 12. The method ofclaim 1 wherein the length of a particular service period is determinedfrom preselected maximum and minimum values, and from specifiedparameter values for the isochronous streams that are active at the timeof determining the particular service period.
 13. Apparatus fortransmitting data frames from a first transmission point in a WLAN to asecond transmission point therein, wherein data frame traffic includesboth isochronous streams and asynchronous bursts, the apparatuscomprising: a first component at a first transmission point forscheduling access times to transmit the data traffic during a successionof service periods, each service period including a specified number ofisochronous epochs and at least one of the service periods including atleast one asynchronous epoch; and a second component at the firsttransmission point acting in cooperation with the first component tosuccessively transmit and receive the isochronous streams duringisochronous epochs and asynchronous bursts during asynchronous epochs.14. The apparatus of claim 13 wherein each of the service periodsincludes two of the isochronous epochs, and further selectively includesone, two or none of the asynchronous epochs.
 15. The apparatus of claim13 wherein the apparatus further incorporates each of the isochronousstreams into one of two stream lists, each stream list corresponding toone of the primary access intervals of each service period.
 16. Theapparatus of claim 13 wherein each of the isochronous epochs includes aprimary access interval, and is adaptable to include an extensionperiod, at least one of the isochronous streams being scheduled foraccess time during each of the primary access intervals, and at leastone of the scheduled isochronous streams being provided with additionalaccess time during each of the extension periods.
 17. The apparatus ofclaim 13 wherein each of the asynchronous epochs includes anasynchronous burst period and a contention period, at least one of theasynchronous bursts being given access time during each of the burstperiods.
 18. The apparatus of claim 13 wherein each of the asynchronousepochs ends before commencement of the next following primary accessinterval.
 19. A computer system for scheduling access times fortransmitting data frames from a first transmission point in a WLAN to asecond transmission point therein, wherein data frame traffic includesboth isochronous streams and asynchronous bursts, the computer systemcomprising: one or more processors; and a computer readable medium,connected to the processors, including instructions to be read by theprocessors and thereby cause the processors to define a succession ofservice periods, each service period including a specified number ofisochronous epochs, adapt at least one of the service periods to includeat least one asynchronous epoch, each asynchronous epoch immediatelyfollowing an isochronous epoch of the same period, and schedule theaccess times for the isochronous streams only during the isochronousepochs, and for the asynchronous bursts only during the asynchronousepochs.
 20. The system of claim 19 wherein each of the service periodsincludes two of the isochronous epochs, and further selectively includesone, two or none of the asynchronous epochs.
 21. The system of claim 19further comprising incorporation of each of the isochronous streams intoone of two stream lists, each stream list corresponding to one of theprimary access intervals of each service period.
 22. The system of claim19 wherein each of the isochronous epochs includes a primary accessinterval, and is adaptable to include an extension period, at least oneof the isochronous streams being scheduled for access time during eachof the primary access intervals, and at least one of the scheduledisochronous streams being provided with additional access time duringeach of the extension periods.
 23. The system of claim 19 wherein eachof the asynchronous epochs includes an asynchronous burst period and acontention period, at least one of the asynchronous bursts being givenaccess time during each of the burst periods.
 24. The system of claim 19wherein the actual access time during one of the primary intervals maystart earlier, but may not end later, than the corresponding scheduledaccess time, and the amount of access time actually used during one ofthe extension periods may not exceed the corresponding scheduled accesstime.